Polymer blends that exhibit some of the properties of a cured elastomer as well as the reprocessability of a thermoplastic resin can be used for a variety of applications and can be processed on equipment designed for thermoplastic materials. The elastomeric characteristics are enhanced if one component of the blend is a vulcanizable elastomer that is wholly or partially crosslinked.
Many of these polymer blends use an organic peroxide curing system. It is known that using an aromatic peroxide increases the degree of crosslinking and produces better compression set compared to other thermoplastic elastomer compositions. However, when the aromatic peroxide dissociates, unreacted radicals form alcohols and ketones, which produce "blooming" at the surface of injection molded and extruded articles.
It is known to use a small amount of tin chloride monohydrate as a catalyst for a phenolic curing system for thermoplastic polyolefins, for example, as disclosed in U.S. Pat. Nos. 5,073,597 and 5,157,081. These two patents disclose a dynamically vulcanized thermoplastic composition comprising (a) a bromobutyl rubber as a primary elastomer, (b) EPM and/or EPDM as a secondary elastomer, and (c) a plastic matrix of crystalline polypropylene. U.S. Pat. No. 5,196,462 discloses the preparation of a fully crosslinked thermoplastic elastomer using a phenolic curing system consisting of a halogenated or non-halogenated phenolic resin and a combination of a metal oxide and a halogen donor such as SnCl.sub.2. 2H.sub.2 O or ZnCl.sub.2. The thermoplastic elastomer comprises (i) a propylene homopolymer or ethylene/propylene copolymer, (ii) an ethylene copolymer or terpolymer rubber, (iii) a semi-crystalline ethylene copolymer that is xylene insoluble at room temperature, and, optionally, (iv) polybutene-1.
However, there is still a need for an additive to prevent blooming when aromatic peroxides are used in the curing system, either alone or in combination with an aliphatic peroxide.